Problem: It takes 36 minutes for 9 people to paint 9 walls. How many minutes does it take 18 people to paint 18 walls?
Answer: Imagine that each person is assigned one wall, and all 9 people begin painting at the same time. Since everyone will finish painting their assigned wall after 36 minutes, it takes one person 36 minutes to paint one wall. If we have 18 people and 18 walls, we can again assign one wall to each person. Everyone will take 36 minutes to paint their assigned wall. In other words, it takes 36 minutes for 18 people to paint 18 walls.